1. Field of the Invention
The invention relates to a linear position sensor and, more particularly, to a linear position sensor for application to, for example, detection of the opening of a throttle valve disposed in an intake air passage of an internal combustion engine.
2. Description of Related Art
A throttle valve disposed in an intake passage of an internal combustion engine is mechanically connected by a wire and the like, to an accelerator pedal that is operated or depressed by a driver. The opening of the throttle valve is determined solely by the amount of depression of the accelerator pedal. In recent years, a so-called electronic throttle control system is also adopted in which the opening and closing of a throttle valve is controlled using an actuator, such as a motor or the like, on the basis of operating conditions of the internal combustion engine.
Such an electronic throttle control system employs a linear position sensor (hereinafter, referred to simply as "sensor") to detect the opening of the throttle valve (throttle opening). The actuator is feedback-controlled so that the throttle opening detected by the sensor becomes equal to a target throttle opening calculated on the basis of engine operating conditions.
In addition to controlling the throttle valve opening to an opening appropriate for the operating conditions of the engine, the electronic throttle control system needs to reliably detect abnormalities, if any occur, in various sensors, in particular, a sensor that detects the throttle opening or a sensor that detects the amount of depression of the accelerator pedal (accelerator depression). An abnormality in such a sensor disables the feedback control. Therefore, if a sensor abnormality occurs, the abnormality must be detected in order to stop the actuator and perform various other operations to cope with the abnormality, for example, forcible reduction of the throttle opening and the like.
For the detection of such abnormalities, a sensor has been proposed which has two output signal generator portions for generating output signals corresponding to throttle openings and allows an abnormality to be detected by comparing the output signals from the two output signal generator portions.
FIG. 11 is a schematic diagram of an example of the aforementioned sensor. A housing (not shown) of the sensor contains a base board (not shown) on which a pair of resistor patterns 100, 101 are provided. The two ends of each of the resistor patterns 100, 101 are electrically connected to a common power supply terminal 102 and a common ground terminal 103, respectively. The electric potential of the power supply terminal 102 is maintained at a supply voltage Ve (for example, 5 V), and the electric potential of the ground terminal 103 is maintained at a ground voltage VGND (0 V).
The sensor further has a rotor (not shown) that is rotatable together with the rotating shaft of a throttle valve (not shown). The rotor has a pair of electrically conductive brushes 104, 105 which contact the surfaces of the resistor patterns 100, 101, respectively. The brushes 104, 105 are connected to output terminals 107, 108, respectively. In this sensor, the brushes 104, 105 and the resistor patterns 100, 101 that are in contact with the brushes 104, 105 constitute two output signal generator portions A, B.
In the sensor constructed as described above, when the rotor is rotated together with the throttle valve, the brushes 104, 105 slide on the surfaces of the resistor patterns 100, 101. As the brushes 104, 105 slide, the voltage signals V1, V2 from the output terminals 107, 108 vary in accordance with the position of the rotor in a rotating direction, that is, the throttle opening.
FIG. 12 shows a graph indicating the relationships between the throttle opening .theta. and the voltage signals VI, V2 (hereinafter, referred to as "output characteristics"). In the graph, a solid line indicates the output characteristic of the output signal generator portion A, and a broken line indicates the output characteristic of the output signal generator portion B.
As indicated by the graph of FIG. 12, the voltage signals V1, V2 increase taking equal values as the throttle opening .theta. increases from a minimum opening .theta.min to a maximum opening .theta.max, if there is no abnormality in the output signal generator portions A, B of the sensor (it should be noted that although, in FIG. 12, the broken line is slightly above the solid line for the purpose of illustration, the two lines are actually on a single straight line).
If, for example, one of the resistor patterns 100, 101 is broken or disconnected, the voltage signals V1, V2 of the output terminals 107, 108 become different in value from each other. Therefore, this sensor is able to detect such a disconnection as a sensor abnormality.
However, the above-described sensor cannot detect an abnormality in some cases, for example, a case where the contact resistance of a connector (not shown) between the power supply terminal 102 and the power source (not shown) or a connector (not shown) between the ground terminal 103 and the ground increases resulting in a decrease in the supply voltage Ve or an increase in the ground voltage VGND.
If the supply voltage Ve of the power supply terminal 102 decreases resulting in a fall of the voltage signals V1, V2 below the normal level, or if the ground voltage VGND at the ground terminal 103 increases resulting in a rise of the voltage signals V1, V2 above the normal level, this sensor is unable to detect such an abnormality. In the sensor, a decrease in the supply voltage Ve or an increase in the ground voltage VGND does not cause the values of the voltage signals V1, V2 to differ from each other.
To solve this problem, a sensor has been proposed (see Japanese Patent Application Laid-Open No. Hei 4-214949) in which the connections between the resistor patterns 100, 101 and the terminals 102, 103 are modified as shown in FIG. 13, so that the rates of change k1, k2 of the voltage signals V1, V2 relative to changes in the throttle opening .theta. (k1=dV1/d.theta., k2=dV2/d.theta.) become positive (k1&gt;0) and negative (k2&lt;0), respectively, as indicated in FIG. 14.
By setting such output characteristics of the output signal generator portions A, B, it becomes possible to detect a decrease in the supply voltage Ve or an increase in the ground voltage VGND as an abnormality. If the supply voltage Ve decreases as indicated by arrows in FIG. 14, the output signal generator portion A outputs a signal indicating a throttle opening .theta.a and the output signal generator portion B outputs a signal indicating a throttle opening .theta.b when the actual throttle opening .theta. is an opening .theta.j. Since there is a difference .DELTA..theta. between the openings .theta.a and .theta.b (.DELTA..theta.=.theta.a-.theta.b&lt;0) as indicated in FIG. 14, a sensor abnormality can be detected on the basis of determination as to whether the absolute value of the difference .DELTA..theta. is greater than a predetermined criterion.
Likewise, if the ground voltage VGND increases as indicated in FIG. 15, a difference .DELTA..theta. exists between the openings .theta.a and .theta.b detected by the output signal generator portions A and B, that is, .DELTA..theta.=.theta.a-.theta.b &gt;0. Therefore, an increase in the ground voltage VGND can also be detected as an abnormality based on the absolute value of the difference .DELTA..theta..
However, this sensor also has problems. If both voltage signals V1, V2 change to an intermediate value between the supply voltage Ve and the ground voltage VGND (that is, (Ve+VGND)/2) due to, for example, short circuit of the output terminals 107, 108 or the resistor patterns 100, 101, the difference .DELTA..theta. becomes "0", so that the sensor cannot detect this abnormality. That is, in a range where the characteristic lines of the output signal generator portions A and B intersect each other, the difference .DELTA..theta. between the openings .theta.a and .theta.b corresponding to the voltage signals obtained by the output signal generator portions A and B becomes "0", so that abnormality detection is impossible.
Moreover, in the aforementioned sensors, repeated slides of the brushes 104, 105 on the resistor patterns 100, 101 will gradually abrade the surfaces of the resistor patterns 100, 101, forming abrasion powder having a predetermined conductivity. If such abrasion powder deposits on the brushes 104, 105, the contact resistance between the brushes 104, 105 and the resistor patterns 100, 101 will increase. The problem of increases in contact resistance is inevitable in the sensors having aforementioned constructions. In these sensors, therefore, it is necessary to allow for fluctuations of the voltage signals due to contact resistance increases while designing a construction to detect sensor abnormalities.
FIGS. 16 and 17 show electric circuit diagrams of the output signal generator portion A. As shown in FIGS. 16 and 17, the output terminal 107 of the output signal generator portion A is connected to an A/D converter of an electronic control unit (not shown), with a pull-down resistor 120 connected therebetween. Although not shown, the output terminal 108 of the output signal generator portion B is also connected to an A/D converter, with a pull-down resistor connected therebetween. In FIGS. 16 and 17, the contact resistance between the brush 104 and the resistor pattern 100 is represented by a resistor r that is disposed between the brush 104 and the output terminal 107.
If the throttle opening .theta. is a maximum opening .theta.max, the brush 104 is at a position on the resistor pattern 100 that is the nearest to the power supply terminal 102 as indicated in FIG. 16. Therefore, the resistance provided between the power supply terminal 102 and the brush 104 by the resistor pattern 100 becomes minimum, so that the voltage fluctuation (voltage fall) .DELTA.V across the contact resistor r relatively becomes maximum.
Conversely, if the throttle opening .theta. is a minimum opening .theta.min, the brush 104 is at a position on the resistor pattern 100 that is the farthest from the power supply terminal 102 as indicated in FIG. 17. Therefore, the resistance provided between the power supply terminal 102 and the brush 104 by the resistor pattern 100 becomes maximum, so that the voltage fluctuation (voltage fall) .DELTA.V by the contact resistor r relatively becomes minimum.
Thus, in the sensor in which the output signal generator portions A, B have opposite output characteristics, the voltage fluctuation by the contact resistance r has a great effect on the voltage signal V2 of the output signal generator portion B if the throttle opening .theta. is relatively small. If the throttle opening .theta. is relatively large, the voltage fluctuation by the contact resistance r has a great effect on the voltage signal V1 of the output signal generator portion A.
Therefore, the aforementioned sensor needs to perform detection of sensor abnormalities while allowing for a maximum voltage fluctuation by the contact resistance r, that is, setting a relatively large criterion value for the detection of a sensor abnormality. As a result, the sensor has a problem of incapability of detecting a sensor abnormality with a high precision.